1. Field of the Invention
The present invention relates to an antireflection structure and an optical device including the antireflection structure.
2. Description of Prior Art
In recent years, various kinds of optical devices in which antireflection processing for suppressing reflection of light is performed to a surface have been proposed. As antireflection processing, for example, processing for formation of an antireflection film including a film having a relatively low refractive index (which will be herein referred to as a “low refractive index film”), a multilayer film in which a low refractive index film and a film having a relatively high refractive index (which will be herein referred to as a “high refractive index film”) are alternately stacked, or like film has been proposed (for example, see Japanese Laid-Open Publication No. 2001-127852 and the like).
However, for formation of an antireflection film including a low refractive index film or a multilayer film, complex processing such as vapor deposition, sputtering and the like need to be performed. Thus, although productivity is low, production costs become high. Moreover, an antireflection film including a low refractive index film or a multilayer film has high dependency on wavelength and incident angle.
In view of the above-described problems, as antireflection processing relatively less dependent on incident angle and wavelength, for example, processing in which fine concave/convex portions are regularly formed on a surface of an optical device with a pitch equal to or smaller than a wavelength of incident light has been proposed (for example, Daniel H. Raguin and G. Michael Morris, “Analysis of antireflection-structured surfaces with continuous one-dimensional surface profiles”, Applied Optics, vol. 32, No. 14, pp. 2582-2598, 1993, and the like). By performing this processing, abrupt change in refractive index in a device interface can be suppressed, so that a refractive index is gradually changed at the device surface. Accordingly, reflection at a surface of an optical device is reduced and a high impingement rate for incident light into the optical device can be achieved.
In National Publication of Translated Version No. 2001-517319, a technique in which fine concave/convex portions are formed on a rough surface is disclosed.